by ‘Spin’ is a fundamental property of fundamental particles such as the electron, which invokes images of a small sphere rotating rapidly on its axis like a planet in a shrunken solar system.
Only it isn’t. It can’t. First, electrons are not spheres of matter, but points described by the mathematics of probability.
But California Institute of Technology philosopher of physics Charles T. Sebens argues that such a particle-based approach to one of the most accurate theories in physics can mislead us.
By framing the basis of matter primarily in the form of fields, he says, certain peculiarities and paradoxes that emerge from a particle-centric view melt away.
“Philosophers tend to be drawn to problems that have been unsolved for a very long time,” says Sebens.
“In quantum mechanics, we have ways of predicting the results of experiments that work very well for electrons and account for a spin, but important fundamental questions remain unanswered: Why do these methods work, and what happens inside an atom?”
For the better part of a century, physicists have wrestled with the results of experiments that suggest that the tiniest bits of reality don’t look or behave like our everyday objects.
Spin is one of those properties. Like a spinning cue ball colliding with the inner wall of a pool table, it carries angular momentum and affects the direction of a moving particle. Yet, unlike the cue ball, a particle’s spin can never increase or decrease its speed—rather, it is always limited to a set value.
To make the fundamental nature of matter even more difficult to picture, consider the fact that an electron’s size is so small that it actually lacks volume. If it were large enough to have volume, the negative charge spread throughout the space would push on itself, tearing the electron apart.
Remarkably, even if we were to be charitable and give the electron as a particle the largest radius experiments would allow, its rotation would overtake the speed of light—which may or may not be a deal breaker on this scale, but for many physicists is enough to dismiss talk about rotating electrons.
One way to make the blanket of fundamental physics a little easier to survey is to describe points of matter as actions embedded in the fabric of a field and then interpret those actions as particles.
Quantum field theory (QFT) does this successfully, weaving together aspects of Einstein’s special theory of relativity, classical field theory and the particle proposition of quantum physics.
It’s not a controversial theory, but there is still debate about whether these fields are fundamental – existing even if the blips rippling through them were to fall silent – or whether particles are the main players representing the vital information and the fields are just a convenient script.
To us, it may seem like a trivial difference. But for philosophers like Sebens, the consequences are worth exploring.
As he explained in a 2019 article featured in Aeon magazine: “Sometimes progress in physics first requires backing up to reevaluate, interpret, and revise the theories we already have.”
This reappraisal of quantum field theory underscores several significant advantages of making fields a priority in physics over a particle-first approach, including a model that re-imagines electrons in ways that can give us better insight into their behavior.
“In an atom, the electron is often depicted as a cloud that shows where the electron can be found, but I think the electron is actually physically spread out across that cloud,” says Sebens.
By being physically spread out through a field rather than confined to a point, an electron can actually rotate in ways that are less mathematical constructs and more a physical description.
Although there would still be no such thing as a small planet in a solar system, at least this spinning electron would be moving at a speed that defies no laws.
Just how this diffuse spread of negatively charged matter resists blowing itself apart is a question Sebens does not have an answer to. But by focusing on the field aspects of a scattered electron, he feels that any solutions will make more sense than problems arising from particles of infinite confinement.
There is a quote that has become folklore in the halls of quantum theorists – “Shut up and calculate.” It has become a saying synonymous with the fantasy landscape of the quantum realm, where images and metaphors cannot compete with the uncanny precision of pure mathematics.
But every now and then it’s important to pause our calculations and indulge in challenging some old assumptions—and maybe even turn around to gain a new perspective on the fundamentals of physics.
This article was published in Synthesis.
